1. Field of the Invention
This invention relates in general to a charge pump, and more particularly, to a charge pump having low static error characteristics.
2. Description of Related Art
Phase-locked loop (PLL) circuits and gain control loops are widely used in electronic systems today. For example, phase-locked loops may be used for clock recovery and frequency synthesis while gain control loops are used to maintain an output signal at a predetermined level. Nevertheless, applications of such circuits continue to make more stringent demands in terms of the noise immunity and static error performance.
More particularly, by way of example, a phase detector may detect any phase difference between an input signal and the output signal of a voltage controlled oscillator (VCO) and generate control signals to minimize the phase difference in a phase-locked loop circuit. The PLL may employ a charge pump circuit which responds to control signals from the phase detector to modulate the amount of charge stored in a loop filter between the charge pump circuit and the voltage controlled oscillator. The output voltage of the loop filter controls the voltage controlled oscillator frequency.
In a phase-locked loop circuit used to synchronize data input with clock signals, the voltage controlled oscillator output constitutes the clock signal which must be matched in frequency and phase to the input data signal. A frequency detector detects the difference in frequency between the voltage controlled oscillator clock output and reference signal. If the voltage controlled oscillator clock frequency is slower than the data signal, the charge pump charges up and increases the voltage controlled oscillator output frequency. If the voltage controlled oscillator clock frequency is faster, the charge pump charges down and decreases the voltage controlled oscillator output frequency. After the frequency matching or acquisition operation is done, a phase detector acts to match the phase of the data signal and clock. The charge pump combines the signal from both the phase detector and frequency detector to drive the voltage controlled oscillator.
It is important to match the charge and discharge currents in the charge pump circuit. A mismatch in the two currents results in static error between the input signal and the output signal of the voltage controlled oscillator. This static error can be a serious problem in many systems. For example, the bit error rate in the read channel of a disk sub-system may be strongly affected by the static error. This mismatch can only be minimized by minimizing the mismatch between the charge and discharge currents from the charge pump circuit.
To achieve high noise immunity in the charge pump, a differential design is needed. Existing differential designs use emitter or source coupled differential pairs to switch current to the loop filter or to the current supply. Thus, they dissipate power in the switches even during the states where no corrections were being issued. Another downfall to these designs is that mismatches in the differential-mode or common-mode feedback stages result in static error due to gain variation of the amplifier. Also, these charge pumps require emitter-coupled logic (ECL) signals instead of complimentary metal-oxide semiconductor (CMOS) inputs.
Other complimentary metal-oxide semiconductor (CMOS) charge pump designs that are known about are single ended designs, have excessive jitter due to current switch overshoot, and lack any common-mode references.
Nevertheless, attempts have been made to provide constant charge and discharge currents with minimal overshoot. For example, U.S. Pat. No. 5,363,066, issued to Chen et al., entitled "FAST CMOS CHARGE PUMP CIRCUIT", and incorporated by reference herein, discloses a charge pump circuit which includes a feedback circuit coupled to the level shifter for comparing the output voltage of the charge pump to predetermined first and second reference voltages. However, Chen still suffers from significant static error.
It can be seen then that there is a need for a charge pump that receives complimentary metal-oxide semiconductor (CMOS) input signals, has high noise immunity, low static error and works at low power supply voltages.